12 For all those who are of a perverse mind, having been set against the Mosaic legislation, judging it to be dissimilar and contrary to the doctrine of the Gospel, have not applied themselves to investigate the causes of the difference of each covenant. Since, therefore, they have been deserted by the paternal love, and puffed up by Satan, being brought over to the doctrine of Simon Magus, they have apostatized in their opinions from Him who is God, and imagined that they have themselves discovered more than the apostles, by finding out another god; and [maintained] that the apostles preached the Gospel still somewhat under the influence of Jewish opinions, but that they themselves are purer [in doctrine], and more intelligent, than the apostles.
Wherefore also Marcion and his followers have betaken themselves to mutilating the Scriptures, not acknowledging some books at all; and, curtailing the Gospel according to Luke and the Epistles of Paul, they assert that these are alone authentic, which they have themselves thus shortened. In another work, however, I shall, God granting [me strength], refute them out of these which they still retain. But all the rest, inflated with the false name of “knowledge,” do certainly recognise the Scriptures; but they pervert the interpretations, as I have shown in the first book.
And, indeed, the followers of Marcion do directly blaspheme the Creator, alleging him to be the creator of evils, [but] holding a more tolerable theory as to his origin, [and] maintaining that there are two beings, gods by nature, differing from each other—the one being good, but the other evil. Those from Valentinus, however, while they employ names of a more honourable kind, and set forth that He who is Creator is both Father, and Lord, and God, do [nevertheless] render their theory or sect more blasphemous, by maintaining that He was not produced from any one of those Æons within the Pleroma, but from that defect which had been expelled beyond the Pleroma.
Ignorance of the Scriptures and of the dispensation of God has brought all these things upon them. And in the course of this work I shall touch upon the cause of the difference of the covenants on the one hand, and, on the other hand, of their unity and harmony.
Source: Adversus haereses (New Advent)